TITLE: Arrhythmia mechanisms in Long QT Syndrome ABSTRACT The long QT syndrome (LQTS) is an inherited or acquired disease characterized by QT interval prolongation and is associated with life threatening arrhythmias and high incidences of sudden cardiac deaths in young adults. Among the known variants of congenital LQTS, majority are caused by mutations in genes associated with cardiac repolarizing potassium currents. Experimental studies have suggested an active role of the cardiac conduction system in initiation and maintenance of arrhythmia observed in LQTS. However direct evidence is lacking and the arrhythmia mechanisms are not well understood. In this proposal, a combination of experiments using adult rabbit cardiomyocytes expressing hERG mutations and advanced multiscale computer modeling will be utilized to study the role of cardiac Purkinje system in arrhythmogenesis in inherited as well as drug-induced LQTS, specifically LQT2. The proposers hypothesize that significant reduction of the repolarization reserve in Purkinje cells in presence of LQTS conditions increases their propensity to early afterdepolarizations and triggered activity. Furthermore, the regions within the His-PS exhibit strong differential sensitivity to hERG mutations resulting into heterogeneous activation patterns in the ventricles, thus initiating arrhythmia. Two novel dominant negative mutations, P632L and S428P, in HERG channels (LQT2) will be experimentally studied in transfected cardiomyocytes. The experimental characterizations will be incorporated into human biophysical computer models at multiple levels of integration? molecular, single cell and three dimensional? to obtain further mechanistic insights. The detrimental effects of regional electrical heterogeneity within the Purkinje system and its differential sensitivity to LQTS mutations as well as drug-induced long QT effects will be studied using three dimensional computer modeling and concurrent experiments. Systematic study will be conducted to investigate the role of Purkinje system network in initiating EAD-induced ectopic activity and in bradycardia-dependent triggered episodes of polymorphic VT. The proposed research has the potential to provide invaluable insights about the disease and may reveal mechanisms of arrhythmogenesis in inherited as well as acquired forms of the disease. Keywords: Long QT syndrome, Purkinje system, arrhythmia, and computer modeling.